<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>4_getSnapShotBeforeUpdate的使用场景</title>
    <style>
        .list {
            width: 200px;
            height: 150px;
            background-color: skyblue;
            overflow: auto;
        }

        .news {
            height: 30px;
        }
    </style>
</head>
<body>
<!-- 准备好一个“容器” -->
<div id="test"></div>

<!-- 引入react核心库 -->
<script src="../js/17.0.1/react.development.js"></script>
<!-- 引入react-dom，用于支持react操作DOM -->
<script src="../js/17.0.1/react-dom.development.js"></script>
<!-- 引入babel，用于将jsx转为js -->
<script src="../js/17.0.1/babel.min.js"></script>

<script type="text/babel">
	class NewsList extends React.Component {

		state = { newsArr: [] };

		componentDidMount() {
			setInterval(() => {
				//获取原状态
				const { newsArr } = this.state;
				//模拟一条新闻
				const news = '新闻' + (newsArr.length + 1);
				//更新状态
				this.setState({ newsArr: [news, ...newsArr] });
			}, 1000);
		}

		getSnapshotBeforeUpdate() {
			return this.refs.list.scrollHeight;
		}

		componentDidUpdate(preProps, preState, height) {
			this.refs.list.scrollTop += this.refs.list.scrollHeight - height;
		}

		render() {
			return (
				<div className="list" ref="list">
					{
						this.state.newsArr.map((n, index) => {
							return <div key={ index } className="news">{ n }</div>;
						})
					}
				</div>
			);
		}
	}

	ReactDOM.render(<NewsList/>, document.getElementById('test'));
</script>
</body>
</html>
